1. Field of the Invention
The present invention relates to an object, a process (including a method and a manufacturing method), a machine, a manufacture, or a composition of matter. In particular, the present invention relates to, for example, a semiconductor device, a display device, a light-emitting device, a driving method thereof, or a manufacturing method thereof. The present invention particularly relates to a semiconductor device, a display device, a light-emitting device, or the like each including an oxide semiconductor.
In this specification, a semiconductor device generally means a device which can function by utilizing electronic characteristics of a semiconductor, and an electrooptic device, a semiconductor circuit, and electric equipment are all included in the category of semiconductor devices.
2. Description of the Related Art
Semiconductor elements, for example, transistors formed using a semiconductor thin film which is formed over a substrate having an insulating surface such as a glass substrate (such transistors are also referred to as thin film transistors or TFTs for the abbreviation) and diodes, are applied to a wide range of semiconductor devices such as integrated circuits (ICs) and image display devices (simply also referred to as display devices). A silicon film is widely known as a semiconductor thin film applicable to such a semiconductor element.
For example, however, a transistor including amorphous silicon has low field-effect mobility, though it can be formed over a larger glass substrate and therefore can be manufactured at low cost. In contrast, a transistor including polycrystalline silicon has high field-effect mobility, but requires a crystallization process such as laser annealing and a large number of manufacturing steps and is not always suitable for a larger glass substrate.
Meanwhile, oxide semiconductors have attracted attention in recent years as novel semiconductor materials. Examples of oxide semiconductors include zinc oxide (ZnO), an In—Ga—Zn oxide, and the like. Techniques are under development to manufacture a transistor including a semiconductor thin film formed using such an oxide semiconductor as a material for a channel formation region (see Patent Document 1).
The use of an oxide semiconductor for a channel formation region makes it possible to manufacture a transistor which has both high field-effect mobility comparable to that obtained with polysilicon or microcrystalline silicon and uniform element characteristics comparable to those obtained with amorphous silicon. Since the transistor has high field-effect mobility, in the case where this is used, for example, in a display device, the transistor can have sufficient on-state current even when having a small area. Thus, an increase in aperture ratio of a pixel and/or a reduction of power consumption of a display device due to the increase can be achieved. An oxide semiconductor film can be formed using a sputtering method and is thus suitable for manufacturing a semiconductor device over a large-area substrate. Manufacturing a semiconductor device over a large-area substrate can reduce the manufacturing cost of the semiconductor device. There is another advantage that capital investment can be reduced because part of production equipment for a transistor including an amorphous silicon film can be retrofitted and utilized.
However, it is known that oxygen is released from an oxide semiconductor in a manufacturing process, so that an oxygen vacancy is formed (see Patent Document 2).